190 GENETICS 



in the germ cells the chromosomes remain entire. This is 

 known to occur in thread-worms (Nematodes) and in cer- 

 tain insects. The relation of these processes to the way 

 bodily development occurs is quite unknown. 



But on the whole the evidence is strong that in most or- 

 ganisms the production of different tissues and organs by 

 different cells is not due to their containing different genes, 

 for cells that certainly contain all the genes produce many 

 different parts in many different cases. The genes act differ- 

 ently in the different cells, depending on the relation of the 

 different cells to each other, so that the different cells, 

 though containing the same genes, produce different tissues 

 and organs (see Chapter X for experimental evidence as 

 to this) . 



The diversity of the different parts of the body seemingly 

 arises mainly, if not entirely, through the fact that different 

 cytoplasmic products of the genes are separated into the 

 different cells. Thus the cells become different in their cyto- 

 plasm. And this different cytoplasmic constitution of the 

 different cells interacts differently in each case with the full 

 set of genes that the nucleus contains, so giving anew dif- 

 ferent products. By a continuation of such action the differ- 

 ent tissues and organs are produced. In each, the cells con- 

 tain the same sets of genes, but differ in their cytoplasm. The 

 differentiations of the body therefore lie mainly in the cyto- 

 plasm, though the diversities in the cytoplasm are brought 

 into existence through the action of the genes. 



Yet it is true that differences may arise within the body 

 of a single individual through a change in the genes of some 

 of the cells. As before seen (Chapter III), if in a female 

 Drosophila one of the cells loses one of the X-chromosomes, 

 that part of the body derived from this cell shows the char- 

 acteristics of the male, while the rest is female. If the X 

 that is lost carries dominant genes, while the remaining X 

 has the corresponding recessive genes, then in that part of 



